1. Field of the Invention
This invention relates to a roll-over suppressing control apparatus for a vehicle which secures the traveling performance of the vehicle and suppresses overturning of the vehicle.
2. Description of the Related Art
When a vehicle turns, a moment in the rolling direction is generated on the vehicle body depending upon the traveling speed or the steering situation and sometimes deteriorates the stability of the vehicle. In such an instance, if the moment in the rolling direction increases excessively, then there is the possibility that roll-over (overturning) may occur with the vehicle. Therefore, a control apparatus which, when there is the possibility that a vehicle may suffer from roll-over, decreases the vehicle speed to suppress the roll-over has conventionally been proposed.
For example, Japanese Patent Laid-Open No. 2000-104582 (hereinafter referred to as Patent Document 1) discloses a control apparatus which interrupts, if it is determined that the roll-over behavior of a vehicle is unstable, fuel supply to the engine and re-starts, after the rolling behavior of the vehicle is stabilized, the fuel supply to the engine in response to the value of the lateral acceleration (lateral G) of the vehicle. With the control apparatus of the configuration described, when the rolling behavior of the vehicle is in an unstable state, the engine torque is reduced so that the wheels may grip the road surface. Then, after the roll state of the vehicle is placed into a stable state, the control of reducing the engine torque is ended in response to the value of the lateral acceleration of the vehicle so that a stabilized state wherein hunting of control does not occur can be established.
While the control apparatus disclosed in Patent Document 1 reduces the driving force of the vehicle through control of the engine torque to suppress the vehicle speed, also an apparatus which applies braking force to the wheels to suppress the vehicle speed has been proposed.
For example, Japanese Patent Laid-Open No. Hei 10-24819 (hereinafter referred to as Patent Document 2) discloses a control apparatus which has a first control mode for controlling the posture of a vehicle based on a turning characteristic (magnitude of the yaw rate) of the vehicle and a second control mode for controlling the rolling (overturning) of the vehicle based on an excessive roll parameter (lightness of the load to a wheel or deceleration of a wheel) of the vehicle. In the first control mode, a difference in braking force is provided between inner and outer wheels to control the yaw moment in the turning or turning round direction of the vehicle. In the second control mode, braking force is applied to individual wheels and the amount of fuel to be supplied to the engine is reduced to control the vehicle speed. With the control apparatus having the configuration described, when the vehicle is in the first control mode, restoration of an appropriate posture of the vehicle can be achieved, and when the vehicle is in the second control mode, the centrifugal force to act upon the center of gravity of the vehicle is reduced to prevent rolling of the vehicle and the driving stability can be maintained well.
In this manner, in the control apparatus of Patent Document 2, in order to prevent rolling of a vehicle, control of applying braking force to the individual wheels is carried out simultaneously in addition to the control of reducing the engine torque disclosed in Patent Document 1.
As described hereinabove, where control of suppressing the roll-over of a vehicle is performed, a control apparatus therefor is generally configured such that a parameter value relating to the rolling behavior of the vehicle is referred to and, if the parameter value representative of the rolling behavior is displaced to the behavior unstable side with respect to a control start criterion value, then the control is started, whereafter the control is ended when the parameter value representative of the rolling behavior is displaced to the behavior stable side with respect to a control end criterion value.
The parameter value representative of the rolling behavior in this instance may be the roll rate or the lateral accelerate (lateral G) generated on the vehicle, and the start or the end of the roll-over suppressing control can be determined using such parameter values.
Incidentally, in the roll-over suppressing control for a vehicle, the brake or the engine output power of the vehicle is controlled. However, the brake or the engine output power of the vehicle should originally be controlled in accordance with the intention (operation) of the driver. Accordingly, the roll-over suppressing control for a vehicle which is carried out independently of the intention of the driver should naturally be carried out only when it is required. In this regard, it is significant to set a control start condition and a control end condition such as a control start reference value and a control end reference value to appropriate values.
In the case of the control end condition, the end of the control is determined from the state of the vehicle while the control is being carried out. However, even if the rolling behavior of the vehicle is suppressed while the control is being carried out, when the control is ended in this state, the rolling behavior of the vehicle may sometimes increase again. In this instance, not only the possibility of the roll-over of the vehicle cannot be reduced, but also control hunting takes place unfavorably.
Accordingly, the control system is desirably configured such that, if the rolling behavior of the vehicle increases after the roll-over suppressing control of the vehicle is ended, then the control is continued, but if the rolling behavior of the vehicle does not increase even if the roll-over suppressing control of the vehicle is ended, then the control is ended rapidly.
Meanwhile, turning traveling of a vehicle which makes an object of roll-over suppressing control of the vehicle can be classified roughly into unsteady (transient) turning upon lane change or emergency avoiding steering which involves sudden changeover of the turning direction (such turning is hereinafter referred to also as lane change turning) and steady turning (hereinafter referred to also as simple turning) such as turning along a loop bridge or the like along which turning in the same direction continues and turning along a moderate S-shaped curve.
In the simple turning, the rolling behavior does not change suddenly. Therefore, if the rolling behavior of the vehicle is suppressed once by the roll-over suppressing control, then even if the control is ended thereafter, the rolling behavior seldom increases again. Therefore, preferably the control end condition when the roll-over suppressing control is proceeding during simple turning is set to such a low condition that the rolling behavior is improved a little so that the control is ended without waiting that the rolling behavior of the vehicle is suppressed by a great amount, that is, the control is ended at a point of time at which the rolling behavior of the vehicle is suppressed to a certain degree.
In contrast, in the lane change turning, since the steering direction is changed over midway, the rolling behavior of the vehicle changes suddenly due to an aftershock, and particularly while the roll rate decreases, the rolling angle itself increases. From this situation, even if the rolling behavior (for example, the value of the roll rate) of the vehicle suppressed once by the roll-over suppressing control, when the control is ended thereafter, the rolling behavior of the vehicle sometimes increases again. Therefore, preferably the control end condition when the roll-over suppressing control is proceeding during lane change turning of the vehicle is set to such a high condition that the rolling behavior is enhanced sufficiently so that the control is ended after it is waited that the rolling behavior of the vehicle is suppressed by a great amount.
Conventionally, the end condition of the roll-over suppressing control is set to a fixed condition irrespective of such types of turning described above. Therefore, if the control end condition is set to such a low condition that the rolling behavior is improved a little in conformity with the condition for simple turning, then if the control is ended upon lane change turning, then the rolling behavior of the vehicle may possibly increase again. On the contrary, if the control end condition is set to such a high condition that the rolling behavior is improved sufficiently in conformity with the condition for lane change turning, then upon simple turning, the roll-over suppression control continues for more than a necessary period of time, which gives an unfamiliar feeling to the driver.